Equations

(Open Calc) Heparin-Induced Thrombocytopenia (HIT) 4Ts Score

Background

1. Heparin-induced thrombocytopenia (HIT): prothrombotic adverse drug reaction caused by heparin
2. Thrombocytopenia is common in hospitalized patients receiving heparin
   • It would be useful to be able to differentiate patients with HIT from those with other etiologies for thrombocytopenia
3. A low pretest clinical score for HIT may aid in ruling out HIT in most situations (high-negative predictive value)
   • The implications of an intermediate or high score vary in different clinical settings

Questions

1. What is the Platelet Count for Thrombocytopenia?
   • Platelet count fall > 50% AND platelet nadir ≥ 20 (2 pts)
   • Platelet count fall 30-50% OR platelet nadir 10-19 (1 pt)
   • Platelet count fall < 30% OR platelet nadir < 10 (0 pts)
2. What is the Timing of Platelet Count Fall?
   • Clear onset between 5-10 days or platelet fall ≤ 1 day; prior heparin exposure within 30 days (2 pts)
   • Consistent with 5-10 days fall, but not clear; onset after day 10 or fall ≤ 1 day; prior heparin exposure 30-100 days ago (1 pt)
   • Platelet count fall < 4 days without recent exposure (0 pts)
3. Are there Thrombosis or other sequelae?
   • New thrombosis confirmed; skin necrosis; acute systemic reaction post-intravenous unfractionated heparin bolus (2 pts)
   • Progressive or recurrent thrombosis; non-necrotizing erythematous skin lesions; suspected thrombosis not proven (1 pt)
   • None (0 pts)
4. What are other possible causes of thrombocytopenia?
   • No other apparent etiology (2 pts)
   • Possible other etiology (1 pt)
   • Definite other etiology (0 pts)

Interpretation

1. Scores 1-3 points = low probability of HIT
2. Scores 4-5 points = intermediate probability of HIT
3. Scores 6-8 points = high probability of HIT

References

1. Lo GK, Juhl D, Warkentin TE, et al. Evaluation of pretest clinical score (4 T's) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings. J Thromb Haemost. Apr 2006;4(4):759-765

Created: December 2015